Keeps breathe working on pre-2.7 versions of Python.

Creating sets in the form ``{'v1', 'v2'}`` is not supported before
Python 2.7. This is the only issue so far, which keeps Breathe from running
in such old environments and has been introduced in commit 55689d4a. The
form ``set(['v1', 'v2'])`` is valid for all Python versions.